A contentious week-long first round of Syrian peace talks has ended with no progress towards ending the civil war.
The government delegation was unable to say whether it will return for the next round in ten days.
Complicating matters further, the United States and Russia clashed over the pace of Syria's handover of chemical arms for destruction.
The US accused Syria of foot-dragging, putting the plan six to eight weeks behind schedule.
However, Russia, an ally of President Bashar al-Assad, rejected this.
US Secretary of State John Kerry said Syrian authorities had no excuse for delays in shipping its poison gas arsenal abroad under a deal reached last year.
Russia said Mr Assad was acting "in good faith" and a 30 June deadline for eliminating the chemical agents remains viable.
UN mediator Lakhdar Brahimi, who has tirelessly pursued a peace deal that other diplomats consider impossible, said the opposition delegation would be back on 10 February.
However, Mr Assad's delegates had told him they would have to check with Damascus before agreeing to return.
"They didn't tell me that they are thinking of not coming. On the contrary, they said that they would come but they needed to check with their capital," Mr Brahimi told a news conference.
Mr Brahimi listed ten simple points that he felt the two sides agreed on in the talks and said he thought there was more common ground than the sides recognised.
But neither side has budged an inch from their main positions.
The opposition wants the talks to focus on a transitional administration it says will remove Mr Assad from power, while the government wants to talk about fighting "terrorism" - a word it uses to refer to all of its armed foes.
"Progress is very slow indeed, but the sides have engaged in an acceptable manner," Mr Brahimi said.
Foreign Minister Walid al-Moualem blamed the lack of tangible results on what he called the immaturity of the opposition delegation and their "threats to implode" the talks, as well as blatant US interference.
He said there was no pressure on his delegation from Russia, but there was coordination between Syria and Russia.
Expectations had always been low for a breakthrough on political issues at the talks.
The talks were the first between Mr Assad's representatives and his opponents in an almost three-year-old conflict that has killed 130,000 Syrians and driven a third of the population from their homes.
The sides also failed to achieve more modest aims, such as an agreement to allow aid convoys into Homs, Syria's third largest city.
Thousands of civilians are trapped in Homs with no access to food or medicine.
"Homs was extensively discussed, although unfortunately there has been no breakthrough yet," Mr Brahimi said.